You may be looking for the Dalek superweapon.

A reality bomb was a weapon created to fight the Time Lords by a race whose entire timeline was erased during one of the early time wars.

After Gallifrey developed time travel, they eliminated any potential temporal rivals by retroactively removing them from existence. One such unnamed race created the reality bomb.

The bomb's main mass, a large ring of matter-disruptors, existed in the physical universe. It extend partially into the Time Vortex, corrupting the control systems of any TARDIS it contacted. A false distress signal was created, appearing to be from a source which the pilot would want to go to as fast as possible. Just before materialisation was complete, the bomb exploded, destroying the TARDIS within the ring.

At least one reality bomb survived the elimination of its creators. It was hidden by dark matter in the Horsehead Nebula for fifty billion years. The Seventh Doctor encountered it, but since Roz Forrester and Bernice Summerfield saw different distress messages than him, he realised what was happening. He managed to prevent the total destruction of the TARDIS, but it was still severely damaged. (PROSE: Sky Pirates!)

During the War in Heaven, Romana III's Gallifrey used carefully-detonated reality bombs to obscure truths from the population. (PROSE: The Ancestor Cell)

Community content is available under CC-BY-SA unless otherwise noted.